1. Field of the Invention
The present invention relates to an aqueous dispersion and, more specifically, to an aqueous dispersion of an ethylene-vinyl alcohol copolymer having excellent dispersion stability, and to a substrate coated with the same.
2. Description of the Prior Art
Ethylene-vinyl alcohol copolymers (hereinafter referred to as EVOH) prepared by saponification of ethylene-vinyl ester copolymers, particularly ethylene-vinyl acetate copolymer, have been attracting much attention as packaging materials and protective covering materials for plastic moldings, metal surfaces, papers, woods, etc. because of their superior barrier properties against gases such as oxygen and their good resistance to oils and chemicals.
In particular, high gas barrier properties are required for those films, sheets, laminates, hollow containers, etc. for packaging foods, that should prevent oxidation and maintain fragrance of the packaged contents. Plain articles comprising soft polyvinyl chloride, such as wall papers, coated fabrics and sheets, require a covering material to prevent the plasticizer used from bleeding out. It is then widely practiced to provide an EVOH layer having superior gas barrier properties, fragrance retention property and good resistance to oils and chemicals as the inner, outer, and/or intermediate layer of the articles to satisfy these requirements in a high degree.
Melt extrusion, injection molding and lamination are widely used for providing such an EVOH layer in general. It has been also proposed to provide an EVOH layer by applying a solution or aqueous dispersion of the EVOH and then drying it. This process has caught attention because of its ability to form a comparatively thin layer, and to coat easily hollow containers or the like having complex shapes and like advantages.
However, the process of applying an EVOH solution has the following problems. High concentration solutions cannot be applied because of their high viscosity. Since the solution uses an organic solvent such as dimethyl sulfoxide or a mixed solvent of water and a large amount of alcohol, the organic solvent used evaporates during film formation process and deteriorates the working environment or requires costly solvent recovery equipment. On the contrary, the process of applying an aqueous dispersion of EVOH is expected to be advantageous over the above described solution-applying process from the standpoint of working environment protection and economy.
For aqueous dispersions of EVOH, for example Japanese Patent Application Laid-open Nos. 101844/1979 and 61430/1981 disclose an emulsified dispersion of a conventional EVOH with a conventional surfactant or protective polymer colloid such as polyethylene oxide, carboxymethylcellulose, hydroxyethylcellulose or polyvinyl alcohol. Studies made by the present inventors have however revealed that the aqueous dispersion of EVOH obtained by these known processes are insufficient in dispersion stability and difficult to use for practical coating purposes.
Japanese Patent Application Laid-open No. 101844/1979 discloses that an aqueous EVOH dispersion is obtained by dispersing a what is known as random-copolymerized EVOH having carboxyl anion group obtained by terpolymerizing ethylene and vinyl acetate with a monomer having carboxylic group, such as acrylic acid or maleic anhydride, with a conventional surfactant as a dispersion stabilizer.
However, to assure sufficient dispersion of the what is known as the random copolymerized carboxyl anion-modified EVOH, it is necessary that a large amount of the carboxyl anion-containing monomer be copolymerized, because of the ionic group being randomly distributed throughout the entire EVOH. Presence of such large amount of the carboxyl anion reduces the crystallinity of the EVOH and increases the molecular structure turbulance, which tends to deteriorate the barrier property of the formed EVOH film.